crop.Circle returns empty image when passing it an image returned from img.SubImage

[meblum]

Calling crop.Circle(cropImage(myImage, myRect), -1, 0) returns a blank image.

Here is the cropImage implementation:

// cropImage takes an image and crops it to the specified rectangle.
func cropImage(img image.Image, crop image.Rectangle) image.Image {
	type subImager interface {
		SubImage(r image.Rectangle) image.Image
	}

	// img is an Image interface. This checks if the underlying value has a
	// method called SubImage. If it does, then we can use SubImage to crop the
	// image. The image types in the std lib do implement it.
	simg, ok := img.(subImager)
	if !ok {
		panic("image does not support cropping")
	}

	return simg.SubImage(crop)
}